1. Field of the Invention
The present invention relates to a vertical organic transistor used as a driving device of a spontaneous light-emitting organic electroluminescent (EL) display.
2. Description of the Related Art
In recent years, full-color displays using organic EL devices have been attracting a great deal of attention because of their potential advantages of (1) the light and compact structure, (2) the increased size of display screen, (3) the reduced fabrication cost, and (4) the capability of various types of light emission, as compared with light-emitting devices using inorganic materials. To realize such a full-color display using organic EL devices as commercially available products, various studies have been made.
Since an organic EL display using organic EL devices can be made thin with high intensity and high response rate, it is expected as a next-generation display device to replace currently wide-spread liquid crystal display devices. However, if an organic thin film transistor (TFT) fabricated with organic semiconductor materials is used as a driving device of an organic EL display, satisfactory driving operations cannot be achieved at the current stage because of high electric resistance and low charge mobility. Therefore, an organic TFT with an improved structure and characteristic is desired.
Organic transistors have been studied since the beginning of the 1980s, and the basic characteristics of organic semiconductor films made of low-molecular compounds and organic semiconductor films made of macromolecular compounds have been examined. However, because of the low electric charge mobility and high electric resistance of organic semiconductors, not so much attention was paid to practical applications in the earlier stage.
In recent years, studies on organic semiconductor films have been actively made to aim at practical use in cellular phones or large-sized display devices of the next generation, taking advantage of the lightness and the flexibility. For example, JPA 10-270712 discloses an organic TFT that achieves the electric charge mobility of 0.52 cm2/V*sec by forming a pentacene film on a highly doped silicon substrate.
An organic semiconductor material includes (1) low-molecular compounds such as pentacene, and a metal complex of phthalocyanine, (2) a short-chain oligomer that contains 3 through 8 monomer units (c3 through c8) of thiophene, and (3) a long-chain polymer such as poly(thiophene), and poly(phenylenevinylene). The long-change polymer is known as a conductive polymer of a π-conjugated system, and electric charges can move along the molecules, the oligomer, and the polymer owing to the overlapped atomic orbits of multiple-bonded adjacent atoms. In addition, depending on the overlapped structure of the molecular orbits of adjacent molecules, electric charges can also move between molecules.
It is known that an organic thin film of a low-molecular compound or short-chain oligomer exhibits the highest electric charge mobility among organic materials. Such low-molecular compound or short-chain oligomer can be deposited as a regularly configured thin film by vacuum evaporation. The regular configuration within the thin film is assumed to produce overlapped atomic orbits, causing electric charges to move between adjacent molecules.
A film of long-chain polymer can be formed by a low-cost process, such as spin coating, or dipping coating because of the soluble characteristic, and is advantageous industrially. However, since the thin film of long-chain polymer has an irregular polymeric configuration, the electric charge mobility is degraded.
In short, there has been no organic semiconductor material having a definitely high mobility found so far.
Under these circumstances, a conventional (lateral type) field effect transistor (FET) may be arranged adjacent to the organic EL device in order to drive the organic EL device. However, merely introducing the conventional FET as a driving device cannot achieve satisfactory characteristics from the viewpoints of operation rate and electric power, because of poor mobility of electric charge.
Therefore, the inventors of the present invention have proposed a vertical organic static induction transistor (SIT) having an improved switching characteristic. See “Schottky Gate Static Induction Transistor Using Copper Phthalocyanine Films”, Kudo, et al., Thin Solid Films 331(1998)51–54. The SIT employs a vertical FET structure as the switching device, which can achieve a large electric current and a relatively high operation rate even though the electric charge mobility is not so high.
The conventional field effect transistor is of a lateral type, which causes electric current to flow in the horizontal direction along the active layer. In contrast, in the vertical SIT, electric current flows in the vertical direction across the active layer. With the vertical structure, (a) the channel length of the transistor can be reduced to or below the thickness of the organic thin film, without requiring a photo-lithography technique, (b) the entire area of the, electrode formed on the surface of the organic layer can be efficiently used, and (c) adverse influence of the roughness of the channel interface effect can be reduced. For these reasons, a large electric current and a high operational rate can be expected even if an organic semiconductor material inferior in electric charge mobility and electric resistance is used as a semiconductor layer. The fabrication process of a composite-type organic light-emitting device, which is the combination of a vertical SIT and an organic EL device, is simple. In addition, since the FET does not prevent occupation of the display regions, the area efficiency can be improved.
FIG. 1 schematically illustrates a static induction transistor (SIT), which is used to explain the operation mechanism of the SIT. In general, the SIT has a semiconductor layer 104 sandwiched between the n+ type source electrode 101 and the n+ type drain electrode 102, with p+ type gate electrodes 103 inserted in the semiconductor layer 104. If a negative electric voltage is applied to the p+ type gate electrodes 103, depletion layers 105 (indicated by the dashed lines) extend from the p+ type gate electrodes 103 into the semiconductor layer 104. As long as the absolute value of the gate voltage is smaller than that of the pinch off voltage at which the depletion layers 105 contact with each other, the SIT is in the ON state. To turn off the SIT, a negative voltage is applied between the p+ type gate electrode 103 and the n+ type source electrode 101 to raise the electric potential level. The electric current ISD flowing between the n+ type source electrode 101 and the n+ type drain electrode 102 is determined by the electric voltage applied to the p+ type gate electrode 103 and the potential barrier produced by the drain voltage VD.
The SIT with this behavior is called a normally-on mode SIT. The normally-on mode SIT has such characteristics that (1) the operation speed is fast because there is no carrier injection from the gate, (2) a large quantity of electric current can be obtained without concentration of electric current, (3) the electric voltage can be controlled at a small driving power, and (4) an unsaturated I/V (current/voltage) characteristic is exhibited.
As an SIT using an organic semiconductor layer, a vertical TFT with a copper phthalocyanine (hereinafter referred to as “CuPc”) layer sandwiched by the source and drain electrodes is known. (See “Device Operation of Schottky Gate Type Static induction Transistor Using Copper-Phthalocyanine Evaporated Films”, Dong Xing Wang, et al., T.IEE Japan, Vol. 118-A, No. 10 (1998), 1166–1171) In this publication, gate electrodes are made as aluminum strips formed by vacuum evaporation, and positioned in the CuPc (organic material) layer.
A composite type organic light-emitting transistor, in which α-NPD and Alq3 are arranged on CuPc, is also know (See “Fabrication of Hybrid Organic Electroluminescence Transistor”, Ikegami, et al., Electronic Information Communication Association, OME200o-20, at 47–51). In this publication, α-NPD (i.e., bis[N-(1-naphthyl)-N-phenyl]benzidine, which is a low-molecular weight arylamine derivative) functions as a hole transport material, while Alq3 (i.e., tris(8-quinolinolato) aluminum complex) functions as a luminous material. The gate electrodes are formed in the α-NPD layer.
In the above-described SIT, a Schottky barrier is created near the interface between the vacuum-evaporated organic molecular film of CuPc and the strips of the aluminum electrodes. The aluminum electrodes are formed by two-spot vacuum evaporation. With the two-spot vacuum evaporation, the aluminum evaporation sources are placed at two spots. By adjusting the distances between the evaporation sources, the evaporation mask, and the substrate, the strips of gate electrode can be formed at a uniform interval. In order to make the strips of gate electrodes function as the SIT gate, the slit width between two adjacent gate electrodes has to be set to the width of the depletion region of the Schottky barrier, which is less than several hundreds angstroms. Ordinary vacuum evaporation cannot realize this slit width. Therefore, by making use of the blur effect of the aluminum under the two-spot vacuum evaporation, strip-like semitransparent aluminum films and aluminum non-existing regions are alternately produced with the slit width corresponding to the depletion width.
With the two-spot vacuum evaporation, the positional relationship between the evaporation sources, the metal mask, and the substrate is geometrically set using the trigonometric ratio. To this end, it is difficult to determine the optimum position, and it is unsuitable for mass production.